With the recent AMP update at Google you may notice increased number of AMP parsing errors in your search console. They look like
The mandatory tag 'html ⚡ for top-level html' is missing or incorrect.
Some plugins, e.g. Add Meta Tags, may alter language_attributes() using 'language_attributes' filter and adding XML-related attributes which are disallowed (see www.ampproject.org/docs/reference/spec#required-markup ) and that causes the error mentioned above.
I have made a fix solving this problem and made pull request for WordPress AMP plugin, you may see it here:
github.com/Automattic/amp-wp/pull/531
The issue with language_attributes() has been fixed in WP AMP plugin version 0.4.2 in a different way, by replacing language_attributes() call with manually created lang and dir attributes.